1. Field of the Invention
The present invention relates to a method of inhibiting corrosion in cooling tower systems and, more specifically, for lowering the pitting rate associated with tuberculation of carbon steel and other corrosion-prone materials to less than the general corrosion rate.
Cooling towers are widely used in the industry to cool water used in heat exchangers, refrigeration units, etc. Commonly, the cooling tower systems employed in such environments are of the recirculating type; that is, the water used for cooling purposes is recycled to the cooling tower for chilling via evaporation. It is common for the cooling tower water to become corrosive from time to time, regardless of the level of sophistication of chemical addition and treatment. During these occurrences, tuberculation can form on the surface of the metal which provides sites for pitting corrosion. The subsequent pitting beneath the tuberculation is the most serious form of corrosion and the primary cause of corrosion-induced equipment failure in cooling systems.
Specifically then, there are two types of corrosion which must be controlled. General, or uniform, corrosion and pitting, or localized, corrosion. General corrosion rate is the measure of the thickness of metal lost. It is measured in thousands of an inch of metal loss per year, referred to as mils per year (mpy). Pitting corrosion is also expressed as mils per year, but refers to depth at a specific site.
Typically, an untreated water system my have a general (uniform) metal loss of 0.060 inches per year (60 mpy). By the addition of corrosion inhibitors, the general corrosion rate can be reduced. In a properly treated cooling system the general corrosion rate will normally be measured at less than 5.0 mpy. The pitting rate is considered to be properly controlled if it is three to five times the general corrosion rate. Both the general and pitting rates can be measured either via metal coupons, or with electrical corrosion measuring instruments.
2. Description of the Background
Historically, a wide assortment of anti-corrosion compositions have been use for corrosion inhibition. For example, heavy metals, such as water-soluble chromium and zinc compounds have been used to virtually eliminate general corrosion and to a certain extent control pitting corrosion. Pitting corrosion, however, is still a serious problem. Since environmental considerations have progressively eliminated the use of toxic, heavy metals, such as chromate and zinc, less effective or more expensive corrosion inhibitors have come into extensive use. For example, it is known that water-soluble molybdates are effective in controlling corrosion and do not present environmental problems. However, molybdates are relatively expensive to use.
As disclosed in U.S. Pat. No. 4,867,944, incorporated herein by reference, effective corrosion inhibition in cooling tower systems can be accomplished by the use of a composition which includes a water-soluble zinc compound, a water-soluble molybdate and an orthophosphate. Similar corrosion inhibitors are also disclosed, for example, in U.S. Pat. Nos. 4,217,216; 4,176,059; 4,017,315; DE No. 2850925 and Japan Kokai JP No. 52/38438 (77/38437). Additionally, an article entitled "Molybdate As A Pipeline Corrosion Inhibitor For Co-Water Slurry Systems", Phys. Metall. Res. Lab. 1986, discloses a composition comprised of molybdate, zinc sulfate and potassium phosphate as an erosion-corrosion inhibitor for steel used in cold water slurries.
Although the use of molybdates, alone and in combination with other corrosion inhibitors such as phosphates, provide more effective general corrosion inhibitors in the sense that certain environmental problems can be alleviated if the molybdates are used without toxic, heavy metals, there is still no known method of effecting control of pitting corrosion to the point where it can be virtually eliminated or at least reduced to a point less than or equal to the general corrosion rate.